In one known type of inertial navigation system, angular rate of rotation about a given coordinate axis is measured by moving an analog accelerometer along an axis normal to the accelerometer's sensitive axis and normal to the axis about which rotation is to be measured. For example, consider a set of X, Y and Z coordinate axes fixed in a body whose rotation rate is to be measured and an accelerometer also fixed in the body and having its sensitive axis aligned along the Z axis. If the angular rotation vector of the body includes a component along the X axis, then periodic motion of the accelerometer along the Y axis will result in a periodic Coriolis acceleration acting in the Z direction that will be sensed by the accelerometer. The magnitude of the Coriolis acceleration is proportional to the rotation rate about the X axis. As a result, the output of the accelerometer (typically a current signal) includes a DC or slowly changing component that represents the linear acceleration of the body along the Z axis and a periodic component that represents the rotation of the body about the X axis. The accelerometer output can be processed, along with the outputs of accelerometers that have their sensitive axes in the X and Y directions and that are moved along the Z and X axes, respectively, to yield linear acceleration and angular rate about the X, Y and Z axes. Such signal processing is described in U.S. Pat. No. 4,445,376, and in U.S. Pat. No. 4,590,801.
In the past, inertial navigation systems of the type described above have employed accelerometers that produce an analog output signal, such as a current signal, having a magnitude proportional to the sensed acceleration. A second known type of accelerometer produces an output signal that has a frequency related to the sensed acceleration. An example of such a frequency output accelerometer is a vibrating beam accelerometer. In a vibrating beam accelerometer, a proof mass is supported by a flexure hinge or the like and by a vibrating beam force sensing element that extends along the accelerometer's sensitive axis. The force sensing element is coupled to a drive circuit that causes the force sensing element to vibrate at its resonant frequency. An acceleration along the sensitive axis causes the proof mass to exert a tension or compression force on the force sensing element. In a manner analogous to a vibrating string, a tension force on the force sensing element causes its resonant frequency to rise, while a compression force on the force sensing element causes its resonant frequency to decrease. The force sensing element can therefore be operated as a force-to-frequency converter that frequency modulates an acceleration signal onto a carrier signal, the carrier signal being the zero acceleration resonant frequency of the force sensing element.
One advantage of vibrating beam accelerometers is the fact that their output signals are inherently digital, and may therefore be conveniently integrated into a microprocessor based inertial navigation system. The use of vibrating beam accelerometers as angular rate sensors, however, requires that a technique be developed for processing the output signals so that accelerations resulting from Coriolis accelerations are separated from linear accelerations of the body in which the accelerometer is housed.